I recently set up 2-factor authentication on my apple account to provide stronger protection of my account.

Around about the same time I noticed that iMessages and calls were not being shared across my multiple apple devices as they had been. I went into settings -> icloud -> <your apple id> -> where there are settings for:

contact information

password & security

devices

payment

I made sure that in "devices" all my devices were showing up (they are listed automatically if signed in against your apple id) - which they were

In password and security I made sure that both my mobile phones were "trusted phone numbers"

I figured that should now do the trick..

It didn't

After googling a bit, turns out you also have to go into settings -> messages -> Send & Receive and in there make sure that all your devices and email addresses are listed. You can also define which number/email messages appear to come from when you compose them; which means each device can initiate from its own number, or if you like, they can all initiate from the same ID.